Two-way radio communication system



Dec. 23, 1947. J. c. E. MITCHELL ETAL 2,433,290

TWO-WAY RADIO COMMUNICATION SYSTEM Filed March 23, 1945 2 Sheets-Sheet'l 05C l LLATUR Grumman "MIMIC FIGJA LBW M55 FILTER 5M I000 Kt VAR. o'sc.

HARMONIC SILIECTOR FIG-2 H AR MONlC SELECTOR INVEN TORS JOHN cIA'M/ roe 5.4 .4

. By mLl/fiMEMUPR/JON ATTOR R'EY 1947- J. c. E. MITCHELL ET AL 2,433,290

TWO'WAY RADIO COMMUNICATION SYSTEM Filed March 23, 1945 2 Sheets-Sheet 2 R E T H M S N A R T l L I 7 I I I 2m: MIXER RF. AMHIFIER MDDALATOR AMPAFIER MIXER,

IN VEN TORS a1 Y Z W m m T NW6 6 a NMM MM Patented Dec. 23, 1947 TWO -WAY RADIO COMMUNICATION SYSTEM John C. E. Mitchell, Montreal, Quebec, and William B. Morrison, Verdun, Quebec, Canada, assignors, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application March 23, 1945, Serial No. 584,392

In Canada January 22, 1945 10 Claims.

Th present invention relates generally to a two-way radio communication system, and more particularly to an improved two-way system wherein the operating or working frequency for both transmission and reception is the same.

Point-to-point communication, between two remote radio stations, for example, is frequently accompanied by difficulty in establishing contact, particularly when no time schedule has been arranged. At the ultra-high frequencies, furthermore, there is difficulty in maintaining an exact frequency of the carrier wave, and elaborate control apparatus is commonly needed at both receiving and transmitting ends. The system according to our present invention overcomes these disadvantages to a large extent, since by its use any station may communicate in both directions with a mate station by simply tuning in a transmission from that mate station.

Accordingly, it is an object of our invention to provide a communication system in which the establishment of a suitable carrier frequency for two-way communication may be determined from either one of two or more stations.

Another object of our invention is to provide a two-way communication system in which adjustment of the receiver at one station to the carrier frequency transmitted from another station automatically adjusts the transmitter at the first station to the same carrier frequency.

A more specific object of our invention is to provide at each of a plurality of similar stations an oscillator system of high stability which is utilized for both transmission and reception and which results in extreme accuracy in frequency setup and in perfect communication between the several stations, so that netting between stations is unnecessary.

Other objects and advantages will be apparent from the following description when read in corn junction with the accompanying drawings, in which Figs. 1A and 13, when placed side by side, represent a diagrammatic block diagram of a preferred embodiment of the invention, and Fig. 2 is a schematic circuit diagram of the harmonic selector utilized in the oscillator section of Fi 1A.

Briefly, the method we employ utilizes a selected one of a plurality of available frequencies as the carrier for transmission in one direction and cillator.

the same carrier for transmission in the other direction. Tuning the receiver at station A, for example, to the carrier frequency transmitted from station B will simultaneously assure perfect tuning of the receiver at station B in response to the carrier which will then be effective at station A. This method is of particular value when the system is applied to military uses, for communication between mobile units, as for example, armored tanks, aircraft, or between mobile and ground stations.

The transmitter-receiver shown schematically in Figs. 1A and 1B represents a single set or station, of which any desired number of similar construction may be utilized for carrying out the method of communication above mentioned. Also, it is essential to the successful operation of our system that a net of several stations maintain an agreed-upon common workin frequency for a given period during which communication is desired. Although any desired frequency range may be employed, the description that follows will be given for a transmitter-receiver set which was designed and successfully operated for a frequency range of 2-8 mc., in one band, and capable of two modes of operation, namely, CW and radio telephone.

Referring now more particularly to Figs. 1A and 1B,. the local oscillator voltage for the re ceiver and the master oscillator voltage for the transmitter are derived from a common oscillator circuit. This circuit consists of a one megacycle (me) crystal-controlled oscillator I, the appropriate harmonic of which is mixed with a 500 to 1000 kc. variable oscillator 2. This mixed signal is used as the local oscillator for the receiver and is 1500 kc. higher than the working frequency (fw). For send or transmit, the above mixed signal is mixed 9, second time with a 1500 kc. signal, which reduces the signal to the working frequency (fw). The 1500 kc. signal is locked-in to the crystal, as will be explained later, and thus the only possible netting errors would come from the 500 to 1000 kc. variable os- As this oscillator only contributes a small percentage of the signal and may be constructed to have maximum stability, practically perfect netting results.

The crystal used in oscillator circuit I has a fundamental frequency of one megacycle and is accurate to 1.01% over a temperature range of 40 to +50 C. The output of the crystal oscillator is coupled to the input of an harmonic generator 3. The output of the harmonic generator is fed to a pair of tuned circuits t and 5, which are tuned to the desired harmonic by a pair of 12-position selector switches 6 and 2', re spectively, which tie-in appropriate padders and trimmers of condenser-networks 8 and 9, as shown more particularly in Fig. 2. The tuned circuits 4 and 5, the switches?) and l and their respective condenser-networks 8 and 0 constitute the harmonic selector it] of Fig. 1A. The selector switches 6 and l are ganged to a common control shaft H, which is brought out through the front panel of the set and terminates in a corn trol knob l2, identified also by the legend Selector switch control. A dial D'! is utilized in corn junction with the control knob i2 in. order to identify the position of the selector switch and is calibrated in 0.5 mo. intervals from 2 to 8 me, as shown. The use of the harmonic generator and the double-tuned circuit gives good selectivity to the selected harmonic.

The selected harmonic is fed to the grid of a first mixer stage l3, which may be constituted by a, single tube or two tubes connected as a balanced mixer, where it is mixed with the 500 to 1000 kc. variable oscillator 2. The variable oscillator is of the well-known Hartley type, the grid being tapped down the coil to increase stability. The variable oscillator 2 is tuned by a variable condenser I l and is temperature compensated. The condenser is adjustable by means of a control knob I l afiixed to the condenser shaft which is represented by the chain line i i". Mounted on this shaft for movement therewith is a dial D2 which is calibrated in one direction from 0 to 500 kc. and in the opposite direction from 500 to 1000 kc., the calibrations being marked off in 10 kc. intervals.

To reduce spurious response the 500 to 1000 kc. signals are fed to the first mixer 13 through a low-pass filter IS. The mixed signal from $3, the output of which is tuned by a variable condenser !3, is fed to the receiver mixer or converter 3-0 (Fig. 1B) through a tuned amplifier it, which serves to attenuate the unwanted sidebands from the mixer, the output of the amplifier being tuned by variable condenser l6.

For sending, the output of the oscillator amplifier iii is in effect switched from the receiver converter to the grid of a second oscillator mixer I! the output of which is tuned by a variable condenser H. There it is mixed with a 1500 kc. signal obtained from a 500 kc. fixed oscillator i0 also of the Hartley type. In order to stabilize the frequency of oscillator N3, the latter is locked-in with oscillator I which is crystalcontrolled. For this purpose there is provided a coupling from the output of oscillator i to the input of oscillator l8. As shown in the drawing this coupling is obtained through the natural capacity 20 created by placing the free end of a conductor 2! close to the output of the crystal oscillator, and connecting the other end through a switch it in the S or send position to the input of oscillator it. This coupling may also be obtained by a directly connected condenser. The third harmonic (i. e., 1500 kc.) of the fixed oscillator I3 is selected by a double-tuned ci.rcuit Z2 and fed to the second mixer i'i through one pole of a double -pole send-receive switch 25'. The difference signal (i. e., the receiver local oscillator frequency minus 1.500 kc.) is fed through a driver or intermediate power amplifier stage 23, tuned by a variable condenser 23', to a power amplifier stage 24 which may be a twin tube or separate tubes, the two sections or tubes being operated in parallel and as class C.

For radio telephone transmission the power amplifier Z4 is plate modulated by a class B modulator stage 25 which may also utilize a twin tube or separate tubes, the two sections of tubes being operated in push-pull. The. modulator is riven by a two stage audio amplifier Z6 and 2?, a microphone being connected to the input of the first amplifier stage 26.

The receiver portion of the set consists of an R. F. amplifier stage 29 which is coupled to a converter or mixer stage 30, the input of which is tuned by a variable condenser 30. The signal from the oscillator above described is fed to the mixer stage by way of a conductor iii, the output of the mixer being tuned to 1500 kc, which is the receiver I. F. or intermediate frequency. The latter is then amplified in one or more I. F. stages 32, the output of which is coupled to a diode detector 33, the rectified signal being amplified in one or more audio stages 3t, and then reproduced in a telephone 35, or other suitable indicating means.

For receiving CW signals, the crystal lock-in 2t, 2! is removed by actuating switch to the R or receive position. The switch it) is operated in conjunction with the double pole switch it, one pole of which disconnects the power supply +B from the plate of the second mixer ii, and the other of which disconnects the 1500 kc. oscillator output from the second mixer and connects it to the I. F. amplifier 32 of the receiver. The 1500 kc. signal is then used as a beat frequency oscillator, the signal of which is fed by way of conductor 36 to the receiver I. F. am itfier 32 provide a beat note the frequency of which may be adjusted by condenser 37.

The several variable condensers above men tioned, namely, [3, l6, ll, 23 and 30 which tune respectively the output of the first mixer 13, the output of the oscillator amplifier if}, the output of the second mixer ii, the output 0- the intermediate power amplifier 23 and the input to the receiver mixer 30, are ganged to a common shaft, represented by the dash line This shaft is provided with a control knob 35 for manually adjusting the condensers and it is also provided with a dial D3 which is calibrated from 2 to 8 me, as shown.

The transmitter channel is coupled to an antenna coil :30 of the rotary type by way of a coupling condenser 45, and the receiving channel is also coupled to the antenna coil by or a coupling condenser 42 and a switch when the R or receive position, the switch es being under control of a keying relay not shown. In the S or send position of this switch, the low end of the antenna coil is connected to ground through a compensating condenser id. The low end of the antenna coil is also connected to a switch arm 05 which is adapted to be uni-controlled, as indicated by the chain line H broken at :c-x, with the switch arms 6 and 'i of the harmonic selector l0. Cooperating with the switch arm 45 are a plurality of contacts, corresponding to those associated with the harmonic selector switch arms 6 and l of Fig. 2. A condenser-network 46 of appropriate trimmers is associated with the contacts for tuning the output of the transmitter channel and the input of the receiver channel to. the indicated frequencies. At the switch positions for the higher frequencies, namely 6, 6.5, '7 and 7.5 mc., sufficient distributed capacity is present in the associated circuits so that the connection of separate trimmers is unnecessary.

The procedure for tuning each the communicating net to any quency is as follows (assuming frequency is to be 3.250 me.)

1. The selector switch control knob I2 is set so that the dial reading of DI is 3, that is, the nearest half-megacycle point below the desired frequency which, in the example given, is 3.250.

2. The variable oscillator dial D2, by the rotation of the variable oscillator control knob I4, is set to the last three significant figures of the frequency, that is, to 250. It is to be noted that this dial is provided with two calibrations, an upper one from to 500 and a lower one from 1000 to 500, as previously mentioned. These calibrations may be of different colors, such as red and black, respectively, for ease of identification. When the desired frequency is in a half megacycle (e. g., between 3 and 3.5 me.) the upper or red calibration is used. When the desired frequency is in a half megacycle immediately below an even megacycle (e. g., between 4 and 3.5 me.) the lower or black calibration is used. In the example given, the desired frequency is in the half megacycle between 3 and 3.5 me. so that the upper or red calibration is used, being set to 250 as shown.

3. Then the dial D3, by rotation of the control knob 39 for the R. F. amplifier circuits, is set to the desired frequency, 1. e., to 3.250 mo.

4. Finally, the antenn tuning coil 40 is rotated to resonance. For this purpose there is provided a meter (not shown) which indicates when the antenna is tuned up by the maximum deflection of the needle. (In an application of W. B. Morrison, Serial No. 584,393, filed concurrently herewith, there is disclosed a system for tuning the antenna tuning coil to resonance in an automatic manner in response to antenna current.)

The common oscillator circuit for the transmitter and receiver of each set provides the facility that kc. channels may be set up from 2 to 8 me. with extreme accuracy, so that a net of such sets may be independently set up to any channel within this range and be in perfect communication with one another, without the requirement of netting. In addition, this oscillator of the sets of prearranged frethat the Working circuit lends itself admirably to automatic tuning to predetermined preset positions by means known in the art. In the actual construction of the combined transmitting-receiving set described herein automatic motor-driven control means was provided for adjusting the variable oscillator 2 and the condenser ganged interconnected by dash line 38, and a stepping relay was provided for the harmonic selector, all to three preset operative frequencies.

The chart below lists the selected harmonic for each position ofthe selector switch I2, the frequency range of the signals in the output of the 1st mixer (which is also the frequency range of the signals fed to the receiver mixer 30) corresponding to each selector switch position, as well as the corresponding range of the working frequency. It should be noted that for certain of the selector switch positions the frequency of the variable oscillator is added tothe selected harmonic for those positions, while for other switch positions the frequency of the variable oscillator .is subtracted from the selected harmonic for those positions.

500- Kc. 1st 1500 s shas 0511011 l'e- 1X0 monic quency Ow Frequency 3 3.5-4 1.5 2 -2.5 5 4 -4.5 1.5 2. 5-3 4 4. 5-5 1.5 s -3.5 5 5 -5.5 1.5 3.5-4 5 5. 5-5 1.5 4- -4.5 7 -5.5 1.5 4. 5-5 5 a. 5-7 1.5 5 -5.5 s 7 -7.5 1.5 5. 5-5 7 7. 5-5 1.5 5 -5.5 9 s -s.5 1.5 5.5-7 8 8.5-9 1.5 7 -7.5 10 9 -9.5 1.5 7.5-5

The trimmers and padders of the condensernetworks 8 and 9 are chosen of such values that the required harmonic is obtained for each selector switch position. It will be noted that for certain positions, such as for 2.5 and 4, the 5th harmonic is selected for both positions, and for 3.5 and 5, the 6th harmonic is selected. In one case, however, the frequency of the variable oscillator frequency 2 is added to and in the other it is subtracted from the same harmonic. Whether the frequency of the variable oscillator is added or subtracted is dependent upon the adjustment of the variable tuning condenser l3 in the output of the 1st mixer. The frequency output of this mixer (Fm) is in each case 1500 kc. (the frequency F5 of fixed oscillator Iii-22) above the working or carrier frequency (Fw) and is also the local oscillator frequency (F1) that is injected into the mixer stage 30 of the receiver, so that at the Fm-being equal to F1 and F0 being equal to F1.

While our present invention has been disclosed as embodied in a particular form it will be understood. that it may be embodied in other and different forms without departing from the spirit and scope of our invention. For example, as will be obvious to those skilled in the art, other selected' harmonics may be chosen, and the range of the variable oscillator and the frequency of the fixed oscillator-may be different.

What we claim is:

1. The method of providing two-way communication between a pair of remote stations, each of which consists of a transmitter and a superheterodyne receiver operating at a fixed intermediate frequency, which consists in generating a fixed frequency, selecting a predetermined harmonic of said fixed frequency, mixing said selected harmonic with a predetermined frequency from a variable oscillator to derive a resultant frequency, feeding said resultant frequency to the mixer stage of the receiver, mixing said resultant frequency withthe frequency of a fixed oscillator to derive the carrier frequency for the transmitter, transmitting said carrier frequency to a remote station, and receiving from said remote station a frequency which is the same as that of the transmitted carrier frequency and which is so related to the frequency fed to the receiver mixer stage that the difference frequency is that of the 76 receiver intermediate frequency.

2. The method of providing two-way communication between a pair of remote stations, each of which consists of a, transmitter and a superheterodyne receiver operating at a fixed intermediate frequency, which consists in generating a fixed frequency, selecting a predetermined harmonic of said fixed frequency, mixing said selected harmonic with any selected frequency from a variable oscillator which is tunable through a predetermined range, deriving a resultant frequency which is included in a corresponding frequency range, feeding said resultant frequency to the mixer stage of the receiver which is also tunable over a corresponding frequency range, mixing saidre sultant frequency with the frequency of a fixed oscillator which is locked with the first fixed frequency generator to derive a carrier frequency which is included in a corresponding frequency range, transmitting said carrier frequency to a remote station, and receiving from said remote station a frequency which is the. same as that of the transmitted carrier frequency and which is so related to the frequency fed to the receiver mixer stage that the difference frequency is that of the receiver intermediate frequency.

3. A two-way communication system operable between a pair of remote stations, each of which consists of a tranmsitter and a superheterodyne receiver operating at a fixed intermediate frequency, comprising means for generating a fixed frequency, means for selecting a predetermined harmonic of said fixed frequency, a variable oscillator, means for mixing said selected harmonic with a predetermined frequency from said variable oscillator, means for feeding a portion of said resultant frequency to the mixer stage of the receiver, a fixed frequency oscillator, means for mixing another portion of said resultant frequency with the frequency of said fixed oscillator to provide a carrier frequency for the transmitter, means for transmitting said carrier frequency to a remote station, and means at the receiver for receiving from said remote station a frequency which is the same as that of the transmitted carrier frequency and which is so related to the frequency fed to the receiver mixer stage that the difference frequency is that of the receiver intermediate frequency.

4. A two-way communication system operable between at least two radio stations, each station comprising a transmitter, a superheterodyne receiver, and an oscillator common to the transmitter and the receiver, said oscillator comprising a fixed frequencyoscillator, an harmonic generator coupled to said fixed frequency oscillator, means for selecting a predetermined harmonic from said generator, a variable frequency oscillator, means for mixing a predetermined frequency from the variable oscillator and the selected harmonic to obtain a resultant frequency which 'is utilized as the local oscillator of thereceiver, a second fixed frequency oscillator, and means for mixing said resultant frequency and the frequency of said seccnd fixed oscillator to obtain a difference frequency which is utilized as the carrier frequency for the transmitter.

5. A two-way communication system operable between at least two radio stations, each station comprising a transmitter, a superheterodyne receiver, and an oscillator common to the transmitter and the receiver, said oscillator comprising a fixed frequency oscillator, an harmonic generator coupled to said fixed frequency oscillator, means for selecting a. predetermined harmonic from said generator, a variable frequency oscillator, means for mixing a predetermined frequency from the variable oscillator and the selected harmonic to obtain a resultant frequency which is utilized as the local oscillator of the receiver, a second fixed frequency oscillator, means for locking the second fixed oscillator to the first mentioned fixed oscillator, and means for mixing said resultant frequency and the frequency of said fixed oscillator to obtain a difference frequency which is utilized as the carrier frequency for the transmitter.

6. A two-way communication system operable between at least two radio stations, each station comprising a transmitter and a superheterodyne receiver which are adapted for radio telephone and CW e t o and an oscillator common to the transmitter and the receiver, said oscillator comprising a fixed frequency oscillator, an harmonic generator coupled to said fixed frequency oscillator, means for selecting a predetermined harmonic from said generator, a variable frequency oscillator, means for mixing a predetermined frequency from the variable oscillator and the selected harmonic to obtain a resultant frequency which is utilized as the local oscillator of the receiver, a second fixed frequency oscillator, means operable when the system is arranged for telephone operation for mixing said resultant frequency and the frequency of said second fixed oscillator to obtain a difference frequency which is utilized as the carrier frequency for the transmitter, and means operable when the system is arranged for CW operation for converting said second fixed frequency oscillator to serve as a beat frequency oscillator, the frequency of which is applied to the receiver I. F. stage.

7. A two-way communication system operable between at least two similar communicate on the same carrier frequency, each station comprising a transmitter, superheterodyne receiver, and an oscillator comand the receiver, said and which is different for each variable oscillator predetermined frequency, a fixed frequency oscillator, and means for mixing said resultant fre- 8. A communication system as defined in claim 7 wherein means are provided for locking the fixed oscillator to the fixed frequency crystalcontrolled oscillator to stabilize its frequency.

9. A two-way communication system operable between at least, two similar radio stations which are adapted to communicate on the same carrier frequency, each statio comprising a transmitter, a superheterodyne receiver, and an oscillator common to the transmitter and the receiver, said oscillator comprising a fixed frequency oscillator, an harmonic generator coupled to said fixed frequency oscillator, means including a manual selector switch for selecting a predetermined harmonic from said generator, 9, variable frequency oscillator having means for manually adjusting its frequency, means for mixing a predetermined frequency from the variable oscillator and the selected harmonic to obtain a resultant frequency which is utilized as the local oscillator for the mixer of the receiver, a second fixed frequency oscillator, and means for mixing said resultant frequency and the frequency of said second fixed oscillator to obtain a difference frequency which is utilized as the carrier frequency for the transmitter, said first and second mixing means and the receiver mixer being provided with manual control means for adjustin their respective output frequencies in unison.

10. A two-way communication system as defined in claim 9 wherein the harmonic selector switch has associated with it a dial calibrated in half megacycles, and the manually adjustable means for the variable oscillator has associated with it a dial calibrated in 10 kc. divisions through its range, and the last mentioned control means has associated with it a dial calibrated for the range of the system in megacycles, the

arrangement being such that in selecting a desired working frequency the harmonic selector switch is set to the nearest half megacycle point below the desired frequency, the variable oscillator control means is set to the second, third and fourth significant figures of the desired frequency, and the last mentioned control means is set to the desired frequency calibration on the dial, which is the sum of the settings to which the first two dials are set.

J. C. E. MITCHELL. WM. '5. MORRISON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,113,419 Young Apr. 5, 1938 FOREIGN PATENTS Number Country Date 486,448 Great Britain June 2, 1938 

